The Financial Services Authority appears to have widened the scope of its investigation into the near-collapse of Royal Bank of Scotland to include the controversial takeover of a US bank in 2004.

The City regulator is in discussions with the bank's major investors and other experts about the fundraising exercises conducted by the bank as long ago as the takeover of Charter One in 2004.

The discussions represent a significant escalation in the FSA's efforts to get to the bottom of how RBS, once one of the world's biggest banks, needed a £45bn rescue by the taxpayer during the financial crisis of 2008.

The FSA provoked an outcry last December when it said it would not publish the findings of its investigation. It was forced to backtrack and instead promised a report in March this year. However, it is not now expected until the end of the year as the regulator continues to gather evidence.

When RBS wanted to buy a stake in Bank of China one year later, investors forced the bank to limit its holding to 5% and a City analyst dubbed the then chief executive, Sir Fred Goodwin, a "megalomaniac who cares more about size than shareholder value".

When the hostile bid for ABN Amro deal was announced in 2007 – a moment now regarded as the beginning of the bank's downfall – the City was stunned as it believed big deals were off the table.

It is understood that the FSA is still speaking to a number of witnesses as it attempts to produce a report into its handling of the bank, which along with Bradford & Bingley and HBOS needed to be rescued during the 2008 banking crisis.

Among those being questioned for the first time are major investors who are being asked for information about fundraisings that took place between 2004 and April 2008, as well as the subsequent fundraising that took place in April 2008, before the October 2008 taxpayer bailout.

The FSA is under pressure to publish two reports into RBS, one explaining how it regulated the bank and the other explaining what went wrong at the Edinburgh-based bank, which is looking at the acquisition of the Dutch bank ABN Amro in 2007 and fundraisings it undertook in 2008 to bolster its balance sheet.

The FSA has already promised that its report into how it supervised RBS would cover 2005 and the end of April 2008. City sources now believe it is looking back even earlier than this although the FSA is thought be putting most of its emphasis into the post-2005 period.

To produce the reports the regulator has been forced to reopen its investigation and enter discussions with new witnesses after the furore caused when the FSA conceded in December it had closed its investigation into RBS without taking any action against former directors, including Goodwin. It promoted a public outcry that eventually led to the Treasury select committee being given a role overseeing efforts by the FSA to publish a report into its findings.

That report is not expected to be published until the end of the year after it has been subjected to scrutiny by "independent reviewers" – lawyer Bill Knight and City grandee Sir David Walker – who will report back to Andrew Tyrie, the MP who chairs the Treasury select committee.

The FSA had not intended to publish a formal report into RBS when it announced in December that it had closed its investigation into the bank and while it has since written a draft report there is still controversy over putting the report into the public domain without permission from current and former directors.

The decision to approach City investors is thought to be part of the FSA's efforts to establish the communications between the bank and its shareholders during a number of fundraisings. City sources believe this could have a bearing on its report into its own handling of the affair and also the so-called enforcement report, looking at decisions made to buy ABN and the internal controls in place.